A permanent magnet field system is conventionally used in many small electric motors and electric generators. In this case, a magnet is often rigidly fitted to a stator or a rotor by means of adhesive. After bonding the magnet to the rotor or the stator, a magnet cover is mounted thereon in most cases. For example, many motors being used for electric power steering systems (to be referred to simply as EPS hereinafter) are provided with a magnet cover that is arranged around the rotor magnet in order to prevent the motor from falling into a locked condition when the magnet comes off or becomes broken.
Known methods for rigidly holding a magnet arranged on the outer periphery of a rotor core or a rotary shaft include those adapted to hold it by molding, using a non-magnetic member. For instance, Patent Document 1 (Jpn. Pat. Appln. Laid-Open Publication No. 5-153745) discloses a method of filling a non-magnetic member between magnets by die-casting. According to the Patent Document 1, magnets are rigidly fitted to a rotary shaft by adhesive and subsequently a ferromagnetic member is arranged around each of the magnets. Then, an aluminum die-casting material is filled between the magnets to rigidly secure the magnets onto a rotary shaft. Patent Document 2 (Jpn. Pat. Appln. Laid-Open Publication No. 9-19091) discloses a method of fixing a magnet to the outer periphery of a rotor core by integral molding, using synthetic resin. With the method of the Patent Document 2, the magnet is mechanically fixed to the rotor core without using any adhesive agent so that the time necessary for the fixing operation is reduced.
Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 5-153745)
Patent Document 2: Jpn. Pat. Appln. Laid-Open Publication No. 9-19091)
However, the method of rigidly fitting a magnet to a rotor core or the like by means of adhesive is accompanied by the following problems. Firstly, the coefficient of linear expansion of the rotor of such an electric rotary machine varies from component to component so that the magnet rigidly held by adhesive can become broken when it is heated to a high temperature level. The magnet, the multilayer core formed by using silicon steel plates, and the adhesive agent of the electric rotary machine normally show respective coefficients of linear expansion that are different from each other. Thus, they expand to respective extents that are different from each other as the temperature changes. Therefore, as the extent of expansion of the magnet becomes remarkably different from those of the adhesive agent and the multilayer core at high temperatures, the magnet is subjected to tensile force due to the different extents of expansion to consequently give rise to a broken magnet.
Secondly, when the magnet is rigidly held by means of adhesive, the bonding strength can be dispersed according to the bonding conditions and the quantity of the applied adhesive. Additionally, the adhesive agent can easily be degraded in a hot environment. Then, it is difficult to maintain the bonding strength to a desired level and hence the quality of the product may not be guaranteed. Furthermore, it is difficult to accurately bond the magnet. Accordingly, it is difficult to provide products with stable characteristics.
Thirdly, when an adhesive agent is used, the manufacturing cost rises because the cost of the manufacturing facility is high and the number of manufacturing steps is large. A process including steps of washing the surface to be bonded (degreasing), applying adhesive, bonding a magnet, hardening the adhesive (drying) and cooling (to the room temperature) is normally used for rigidly holding a magnet by means of adhesive. Then, a large number of facilities including a washing facility, an adhesive application facility and a hardening facility are required. Additionally, many man-hours are required because the manufacturing process involves a large number of steps, and the cooling step is particularly time consuming. The manufacturing cost rises as the investment for facilities and the number of man-hours increase, and improvements of the problems are called for.
While the Patent Document 2 discloses a magnet fixing method that does not use any adhesive agent, it requires a metal mold for rigidly holding the magnet by molding resin. In other words, the investment for facilities required for using this method may not differ significantly from the investment for facilities necessary for rigidly holding the magnet by adhesive so that the manufacturing cost is also high. Additionally, a specifically devised metal mold is required to hold a magnet at a predetermined position in the metal mold without using adhesive to consequently raise the cost of the metal mold so much.
The object of the present invention is to provide a magnet fixing structure that can accurately fix a magnet to a rotor core or the like at low cost.